This invention relates, in general, to formation of semiconductor films and, more particularly, to formation of epitaxial films.
Fabrication of semiconductor integrated circuits entails a multitude of processing steps including epitaxial film formation, dielectric film formation, photolithography, etching, impurity doping, passivation, packaging, and testing. Manufacturers of semiconductor integrated circuits are continually refining and optimizing these processing steps in an effort to improve the performance of their products as well as decrease manufacturing costs and cycle times. Many semiconductor manufacturers are turning to silicon-germanium (Si-Ge) heterostructure semiconductor technology as an alternative to silicon semiconductor technology. Advantages of Si-Ge heterostructure devices over their silicon counterparts include a lower bandgap energy, a higher emitter injection efficiency, reduced charge storage in the emitters of bipolar transistors, and a lower base transit time.
A critical step in the formation of Si-Ge heterostructure devices is formation of an epitaxial film or layer. Typically, a Si-Ge epitaxial film is formed at temperatures below approximately 800.degree. C. by techniques such as molecular beam epitaxy (MBE) and ultra-high vacuum chemical vapor deposition (UHV/CVD). It is preferable that Si-Ge epitaxial film formation be performed at temperatures below approximately 800.degree. C. to prevent thermal relaxation of a strained Si-Ge epitaxial film. However, at these temperatures the silicon and germanium in source gases comprising these elements are selectively deposited on semiconductor substrate material and not on a dielectric material overlying portions of the semiconductor substrate material; a phenomenon which precludes formation of a blanket Si-Ge epitaxial film. Additionally, a single silicon source gas, such as silane, is used in these systems.
Furthermore, any contaminants such as oxygen and water vapor that may be incorporated into the semiconductor material during deposition would degrade material quality. To prevent incorporation of contaminants, Si-Ge heterostructure epitaxial growth is carried out, for example, in UHV/CVD reactors, wherein a system growth pressure of approximately 1.times.10.sup.-3 millimeters of mercury (mm Hg) may be achieved. A system growth pressure of approximately 1.times.10.sup.-3 mm Hg requires a background pressure of lower than 1.times.10.sup.-8 mm Hg. Pumping a UHV/CVD reactor chamber to pressures of approximately 1.times.10.sup.-8 mm Hg is time consuming and requires expensive and fragile equipment such as turbo-molecular pumps and the like.
Accordingly, it would be advantageous to have a method for forming a Si-Ge epitaxial film on both semiconductor and dielectric materials using more than a single source gas for the silicon and using chemical vapor deposition reactors other than those employing ultra-high vacuum. It would be further advantageous to have a method that is easily integrated into heterostructure processes, has reduced loading effects, and has the capability of producing a heavily doped epitaxial layer.